This invention relates to an apparatus for protecting solid state electrical circuits in a grounded enclosure from transients induced on lines coming into the enclosure when there is a lightning strike, while also maintaining low leakage electrical isolation when a high test potential ("hi-pot test") is applied between the electrical circuits and the grounded enclosure.
Known protection circuits couple between each line and the grounded enclosure. Each protection circuit includes a gas filled surge arrestor coupled between two conductors (line to enclosure). A current limiting impedance, such as a resistor or inductor, in series with a solid state transient suppressor, such as a metal oxide varistor (MOV), are also coupled between the two conductors. This impedance limits current to the transient suppressor and allows sufficient voltage to build up to fire the surge arrestor.
The surge arrestor shunts large currents (many thousands of amps); however, it is slow to fire or turn on. The transient suppressor responds more rapidly; however, it can shunt only lower levels of current.
When there is an induced transient, the transient suppressor will conduct before the gas filled surge arrestor and limit the potential applied to the protected circuit to a level compatible with the maximum rating of the protected circuit. The potential drop across the impedance allows enough potential across the surge arrestor to fire it, which then shunts larger currents.
To avoid undesired circulation of ground currents in circuit conductors and avoid coupling energized circuit conductors to the enclosure, electrical isolation is desired between the circuitry and the enclosure. This isolation is tested by connecting all circuit lines together and imposing a high potential, typically 500 VAC or 700 VDC, between the lines and the enclosure which is normally grounded. Leakage current is measured with the high potential applied and is required to be at a low level, typically 5 milliamperes (mA) or less.
When the hi-pot voltage test is applied to a field mounted instrument having circuitry which includes protective components, these protective components will conduct and shunt current to the enclosure. Therefore, the leakage will exceed the 5 mA leakage current specification. In the past, hi-pot tests were done on instruments with the lightning protection circuits disconnected, and then the lightning protection circuits were reconnected before placing the instrument in service. New test methods require that the lightning protection circuits remain connected while the hi-pot test is performed. The existing lightning protection circuits, however, produce too much leakage through the transient suppressor, and the instrument does not meet the leakage standard.
Thus, there is a need for protection circuitry which will not only better protect the field mounted instruments from an induced transient, but will also ensure that the field mounted instruments will meet leakage limits during high potential voltage testing.